1. Field of the Invention
The present invention relates to an optical irradiation apparatus, an image reading apparatus, and an image forming apparatus. More particularly, the present invention relates to an apparatus having a light guide which is made of a transparent material and is capable of causing light that enters the light guide from a light source-to exit the light guide in a specific direction.
2. Discussion of the Background
A conventional optical irradiation apparatus is used as an optical irradiator that irradiates a manuscript in an image reading apparatus, such that a reflecting image of the manuscript is read by using, for example, a photo acceptance element such as a charge coupled devices (CCD), a complementary metal oxide semiconductor (CMOS), etc. In a case an image reading apparatus reads a color image, light reflecting from the manuscript is generally received by individual photo acceptance elements in colors of red (R), green (G), and blue (B). FIG. 1A illustrates light reflecting from the manuscript to the CCD. The figure includes a manuscript M, a first mirror 19, a second mirror 20, a third mirror 21, an image formation lens 16, and a CCD 17. As shown in FIG. 1A, each photo acceptance element corresponding to each color is arranged so that the positions may differ from each other. Therefore, the received light on each photo acceptance element includes the light reflected from various points of the manuscript. Then, in a direction (i.e., a horizontal direction in FIG. 1A) of the manuscript corresponding to a direction of a row of the photo acceptance elements, it is necessary to improve the quality of reading the image that the intensity of the light be evenly irradiated from the optical irradiation apparatus. Specifically, w and b are defined as shown in FIG. 1B, where w is a width of each photo acceptance element (in the direction of a row of photo acceptance elements), and b is a distance between the center of the photo acceptance element. p, as used below, is a reduction ratio by the optical system from the manuscript to each photo acceptance element. A width X on the manuscript, which needs to be irradiated with evenly intense light (in the horizontal direction in FIG. 1B), is set to (w+bx2+v)/p. “v” is a parameter suitably set in consideration of errors, such as a manufacture error.
In order to irradiate a width X of the manuscript with evenly intense light, for example, an apparatus using a cylinder type xenon lamp as a light source which is arranged so that its longitudinal direction may intersect perpendicularly to the direction of the width X is used as the optical irradiation apparatus. However, in recent years, there has been a demand to save energy and increase the reliability of the image reading apparatus. The xenon lamp consumes a lot of power and the calorific value is great. Therefore, a light source with smaller power consumption and calorific value than the xenon lamp is desired. As such, for example, a light emitting diode (LED) may be used as the light source. However, compared with the xenon lamp, the optical irradiation intensity of the LED is generally small. Therefore, if the LED is simply used as the light source, it is difficult to irradiate light with a sufficient intensity in the width X on the above-mentioned manuscript.
The conventional optical irradiation apparatus, which has a transparent light guide between a light source and a manuscript, is known. The light guide leads the light from the LED toward the manuscript. If the optical irradiation apparatus equipped with such a light guide is used, it becomes possible to concentrate the radial light from the LED to the narrow domain of the width X on the above-mentioned manuscript. Therefore, if the light guide is used, even if the LED that has a small irradiation intensity etc. is used as the light source, it becomes possible to irradiate light with large intensity at the portion of the width X.
If the LED which irradiates light radially is used as light source, in order to concentrate the light in the portion (irradiation target domain) of the width X on the manuscript for intensifying the light in the target domain, it is important to lead the incident light with the light guide as much as possible to the exit plane of the light guide. In order to realize this, it is necessary to prevent the incident light from exiting the light guide before the light reaches the exit plane.
FIG. 2 illustrates incident light entering the light guide. This figure is seen from the direction which intersects perpendicularly to the width X on the above-mentioned manuscript shown in FIG. 1A.
Although a part of the incident light, which entered from an incidence plane 431a of a light guide 431, may pass through the light guide to an exit plane 431b, much of the light reaches a connecting plane 431c first. According to incidence angles (angle with the normal line of the connecting plane 431c) θ1 and θ2 to the connecting plane 431c, a part of the light penetrates the connecting plane 431c. For example, an incidence light L1 does not penetrate the connecting plane 431c. The connecting plane 431c reflects the light as L1′, and a part of an incident light L2 penetrates the connecting plane 431c as L2′. In detail, the incident light L2 with the incidence angle θ2 smaller than a critical angle α to the connecting plane 431c penetrates and exits through the connecting plane 431c to the outside as L2′, and the incidence light L1 with the incidence angle θ1, which is equal to or larger than the critical angle α, to the connecting plane 431c reflects on the connecting plane 431c as L1′ and finally exits from the exit plane 431b. 
FIG. 3 shows a conventional optical irradiation apparatus capable of preventing the incident light from exiting from the light guide before the light reaches the exit plane of the light guide.
In the conventional optical irradiation apparatus, as shown in FIG. 3, which has an LED 32 as light source and an incidence plane 531a of a light guide 531 that is formed in a convex shape so that it makes the incidence angle to a connecting plane 531c greater than the incidence angle when compared to when the incidence plane is flat. This conventional optical irradiation apparatus prevents the incident light from the incidence plane 531a from passing through the connecting plane 531c to the outside, by reflecting the light incident on the connecting plane 531c. If the incidence plane is flat, the incidence angle to the connecting plane 531c is smaller than the critical angle, so that the light may penetrate the connecting plane 531c to the outside of the light guide.
Further, in the above-mentioned conventional optical irradiation apparatus, a reflective part 533 is formed by vapor-depositing aluminum on the external surface of the connecting plane 531c. The incident light which pass through the connecting plane 531c to outside, even if the incidence plane 531a is formed in the shape of convex, t may be returned to the inside of the light guide because the light reflects on the reflective part 533.
Thus, in the above-mentioned conventional optical irradiation equipment, by forming the incidence plane 531a of the light guide 531 in a convex shape, and forming the reflective part 533 on the external surface of the connecting plane 531c, the conventional optical irradiation equipment prevents the incident light from passing through the incidence plane 531a and exiting to the outside of the light guide before the light reaches the exit plane of the light guide.
However, since it is difficult to manufacture the incidence plane 531a of the light guide 531 in a convex shape compared to a flat incidence plane, the manufacturing costs of the convex shaped incidence plane are high. In addition, if the reflective part 533 is formed on the external surface of the connecting plane 531c, additional costs are incurred for the reflective material, and process affixing the reflective part 533 to the external surface of the connecting plane 531c. Therefore, the manufacturing costs of the conventional irradiation apparatus are high. Thus, in the above-mentioned conventional optical irradiation apparatus, although it may prevent the incident light from exiting through the incidence plane 531a to the outside of the light guide before the light reaches the exit plane of the light guide, there is a problem that the manufacturing cost is high.